1. Field of the Invention
The present invention relates in general to ceramic dielectric compositions. In particular, the invention relates to ceramic materials for multilayer ceramic devices such as multilayer ceramic circuit boards. The ceramic compositions can be sintered at low temperatures to produce dense bodies having low dielectric constants and low dielectric losses.
2. Description of the Related Arts
Recently, multilayer ceramic circuit boards have come into use for mounting integrated circuits to reduce the signal propagation delay.
Conventionally, alumina is generally used as an insulating material for a multilayer ceramic circuit board. One major drawback of the alumina multilayer ceramic circuit board is its greater signal delay, a result of the high dielectric constant of alumina (10 at 1 MHz). Furthermore, the sintering temperature of alumina is so high (1,600.degree. C.) that a refractory metal such as tungsten or molybdenum must be used as the wiring conductor material for the multilayer ceramic circuit board. However, since the electrical resistivity of tungsten and molybdenum are comparatively high, these metals are not satisfactory for high speed integrated circuits.
Multilayer ceramic circuit boards using low-dielectric ceramic material have been developed to increase the electrical signal propagation speed as well as the reliability. In addition, the use of highly conductive metals such as gold, silver, and copper is effective for lowering the wiring conductor resistance. However, this requires the ceramic material to be sintered at a temperature below the melting points of these metals.
A method for producing multilayer ceramic circuit boards for use with copper conductors is described in U.S. Pat. No. 4,642,148 issued to Kurihara et al. There is disclosed a ceramic composition comprising 10-75% by weight of alpha-alumina, 5-70% by weight of non-crystalline quarts (fused silica), and 20-60% by weight of borosilicate glass. The sintered ceramic composition has a dielectric constant ranging from 4.8 to 9.6.
U.S. Pat. No. 4,672,152 issued to Shinohara et al. describes a multilayer ceramic circuit board in which a ceramic insulating layer is prepared from a mixture of 50-95% by weight of crystallized glass and 5-50% by weight of a ceramic filler. The sintered ceramic insulating layer has a dielectric constant between 5.1 and 6.0, and a flexural strength above 150 MPa. The crystallized glass consists of 5-20% by weight of lithium oxide, 60-90% by weight of silicon dioxide, 1-10% by weight of aluminum oxide, and 1-5% by weight of alkaline metal oxide other than lithium oxide. The ceramic filler is selected from the group consisting of silicon dioxide, .beta.-eucryptite (LiAlSiO.sub.4), and aluminum oxide.
U.S. Pat. No. 3,926,648 issued to Miller et al. discloses a sintered glass-ceramic body containing hexagonal cordierite as the crystalline phase. The glass composition exhibits a dielectric constant of 5.2 and a coefficient of thermal expansion between 1-2.times.10.sup.-6 K.sup.-1.
U.S. Pat. No. 4,755,490 issued to DiLazzaro describes a low firing temperature ceramic material having a dielectric constant between 4.5 and 6.1 and a coefficient of thermal expansion between 3.9-4.2.times.10.sup.-6 K.sup.-1. The ceramic material is provided from a mixture of 10-50% by weight of alumina, 0-30% by weight of fused silica, and 50-60% by weight of a frit composed of 4% by weight of CaO, 12% by weight of MgO, 29% by weight of B.sub.2 O.sub.3, and 42% by weight of SiO.sub.2. The mixture has a minimum sintering temperature in the range of 850-1000.degree. C.
U.S. Pat. No. 4,788,046 issued to Barringer et al. discloses glass-ceramic packages for integrated circuits by co-sintering a glass-ceramic composite and Cu, Ag, Pd, or Au at a low sintering temperature. The glass-ceramic composite is provided by coating glassy compounds onto ceramic particles. The composite with the lowest dielectric constant (4.5) is obtained by using quartz and borosilicate glass, which exhibits a thermal expansion coefficient greater than 5.5.times.10.sup.-6 K.sup.-1.
U.S. Pat. No. 4,879,261 issued to Burn discloses a low dielectric composition for making a ceramic dielectric body having a dielectric constant less than 5.0. The composition is a mixture of finely divided particles consisting essentially of 70-85% by weight of silica and 15-30% by weight of zinc borate flux. The composition can be used to make green tape and multilayer devices having internal copper conductors such as multilayer capacitors and multilayer interconnects.
U.S. Pat. No. 4,654,095 issued to Steinberg describes a low firing temperature ceramic material having a dielectric constant between 6.5 and 8.1 and a coefficient of thermal expansion between 4.5-7.0.times.10.sup.-6 K.sup.-1. The ceramic material is made of a mixture of 50-70% by weight of noncrystallizable glass, and 50-25% by weight of refractory oxides including alumina, mullite, cordierite, CaZrO.sub.3, forsterite, ZrO.sub.2 and mixtures thereof. The ceramic mixture has a minimum sintering temperature in the range of 850-1000.degree. C.
U.S. Pat. No. 4,655,864 issued to Rellick discloses a low firing temperature ceramic material having a dielectric constant between 6.5-8.1 and a coefficient of thermal expansion between 4.5-7.0.times.10.sup.-6 K.sup.-1. The ceramic material is made of a mixture of 50-70% by weight of noncrystallizable lead calcium aluminum borosilicate glass, and 50-25% by weight of refractory oxides including alumina, alpha-quartz, fused silica, CaZrO.sub.3, and mixtures thereof. The noncrystallizable lead calcium aluminum borosilicate glass has a composition by weight of 56.5% SiO.sub.2, 17.2% PbO.sub.2, 9.1% Al.sub.2 O.sub.3, 8.6% CaO, 4.5% B.sub.2 O.sub.3, 2.4% NaO and 1.7% K.sub.2 O. The ceramic mixture has a minimum sintering temperature in the range of 850-1000.degree. C.
U.S. Pat. No. 5,206,190 issued to Jean et al. discloses a low firing temperature ceramic material having a dielectric constant about 4.5 and a coefficient of thermal expansion between 2.5-4.0.times.10.sup.-6 K.sup.-1. The ceramic material is made of a mixture of 40-50% by volume of noncrystallizable borosilicate glass, and 50-60% by volume of cordierite. The noncrystallizable borosilicate glass has a composition by weight of 60-80% SiO.sub.2, 20-30% B.sub.2 O.sub.3, 0.1-3% CaO, and 0.1-3% selected from the group of K.sub.2 O, Na.sub.2 O and Li.sub.2 O and mixtures thereof. The ceramic mixture has a minimum sintering temperature in the range of 850-950.degree. C.
U.S. Pat. No. 5,260,119 issued to Jean et al. discloses a low firing temperature ceramic material having a dielectric constant about 4.2 and a coefficient of thermal expansion between 3-4.times.10.sup.-6 K.sup.-1. The ceramic material is made of a mixture of 20-50% by weight of noncrystallizable borosilicate glass, and 25-75% by weight of high silica glass containing 95-98 wt % silica. The noncrystallizable borosilicate glass has a composition by weight of 60-80% SiO.sub.2, 19-30% B.sub.2 O.sub.3, 0.1-4% CaO, 0.1-4% Al.sub.2 O.sub.3 and 0.1-4% selected from the group of K.sub.2 O, Na.sub.2 O and Li.sub.2 O and mixtures thereof. The high silica glass has a composition of 0-1 wt % Al.sub.2 O.sub.3 and 3-5 wt % B.sub.2 O.sub.3 and 95-98 wt % SiO.sub.2. The ceramic mixture has a minimum sintering temperature in the range of 850-1000.degree. C.
U.S. Pat. No. 5,786,288 issued to Jean discloses a low firing temperature ceramic material having a dielectric constant of 4.8-5.3 at 1 MHz and a linear thermal expansion coefficient of 4-5.1.times.10.sup.-6 K.sup.-1 in the temperature range of 25-200.degree. C. The ceramic material is made of a mixture of 10-90 wt % Mg--Al--Si glass, and 10-90 wt % Ca--Al--B--Si glass. The Mg--Al--Si glass which forms crystalline cordierite during sintering has a composition by weight of 15-25% MgO, 20-30% Al.sub.2 O.sub.3 and 40-60% SiO.sub.2. The Ca--Al--B--Si glass which forms crystalline feldspar during sintering has a composition of by weight of 15-25% CaO, 15-25% Al.sub.2 O.sub.3, 5-15% B.sub.2 O.sub.3 and 40-60% SiO.sub.2. The ceramic mixture has a sintering temperature about 875.degree. C.
U.S. Pat. No. 5,821,181 issued to Bethke et al. discloses a low firing temperature ceramic material having a dielectric constant between 7.5 and 10.0 and a high-Q of at least about 500. The ceramic material is made of a mixture of 30-70% by weight of glass, and 30-70% by weight of refractory oxides including alumina and at least one modifier selected from the group consisting of TiO.sub.2, SrTiO.sub.3 and CaTiO.sub.3. The glass has a composition by weight of 35-67% SiO.sub.2, 10-25% B.sub.2 O.sub.3, 5-25% of at least one of MgO, CaO, SrO and BaO, and 2-10% of at least one of K.sub.2 O, Na.sub.2 O and Li.sub.2 O. The mixture has a sintering temperature about 875.degree. C.